The present invention relates to an oscillation circuit and a semiconductor integrated circuit including the same, and in particular, relates to an oscillation circuit for outputting an oscillation signal having an accurate frequency and a semiconductor integrated circuit including the same.
In recent years, oscillation circuits for outputting oscillation signals having accurate frequencies have been required. Related techniques are disclosed in Japanese Unexamined Patent Publication No. 2007-243922 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2006-86997 (Patent Document 2).
An oscillation circuit disclosed in Patent Document 1 includes first and second capacitors, first and second comparison circuits, an RS flip-flop circuit, and first and second charge/discharge control circuits.
The first and second capacitors are charged or discharged by current generated by a constant current source. The first comparison circuit compares a first voltage V1 according to the amount of electric charge stored in the first capacitor and a first reference voltage Vst, and outputs a first signal indicating that the first voltage V1 has reached the first reference voltage Vst. The second comparison circuit compares a second voltage V2 according to the amount of electric charge stored in the second capacitor and a second reference voltage Vst, and outputs a second signal indicating that the second voltage V2 has reached the second reference voltage Vst.
The RS flip-flop circuit is set by one of the first signal and the second signal, and reset by the other. The first charge/discharge control circuit charges the first capacitor when the RS flip-flop circuit is set, and discharges the first capacitor when the RS flip-flop circuit is reset. The second charge/discharge control circuit charges the second capacitor when the RS flip-flop circuit is reset, and discharges the second capacitor when the RS flip-flop circuit is set.
Thereby, according to the description in Patent Document 1, the oscillation circuit outputs an oscillation signal having a stable frequency even when noise occurs.
An oscillation circuit disclosed in Patent Document 2 includes an oscillator, a frequency-voltage converter, a difference detector, and an integrator, which are coupled in a closed loop. The oscillator generates an output signal which oscillates at a frequency according to a control signal. The frequency-voltage converter generates a detection signal having a voltage according to the frequency of the output signal. The difference detector generates a difference signal indicating the difference between the detection signal and a reference signal. The integrator generates a control signal by integrating the difference signal.
Further, the oscillator has a relaxation signal generation circuit for generating a relaxation control signal by relaxing a change in the control signal and an output signal generation circuit for generating an output signal which oscillates at a frequency according to the relaxation control signal.
Thereby, according to the description in Patent Document 2, the oscillation circuit generates a high-frequency output signal at low power consumption with stability and accuracy. Further, to suppress an increase in power consumption and a reduction in frequency accuracy, the oscillation circuit is configured without a comparator.